Presence detection system



A ril 13, 1965 w. e. PETTITT PRESENCE DETECTION SYSTEM Original FiledNov. 10, 1960 :5 Sheets-Sheet 2 INVENTOR. BY W.G. PETTITT NQE HISATTORNEY April 1965 w. e. PETTITT 3,178,685

PRESENCE DETECTION SYSTEM v Original Filed Nov. 10, 1960 3 Sheets-Sheet3 FIG. 3.

IN V EN TOR.

W.G. PETTITT HIS ATTORNEY United States Patent 3,178,685 PRESENCEDETECTION SYTEM Walter G. Pettitt, Rochester, N.Y., assignor to GeneralSignal Corporation, a corporation of New York Continuation ofapplication Ser. No. 68,532, Nov. 10, 1960. This application Dec. 6,1963, Ser. No. 329,349 7 Claims. '(Cl. 340-438) This invention relatesto a presence detection system, and, more particularly, pertains to thedetection of a conveyance within a defined area for causing suitableindications thereof.

This is a continuation of my copending application Ser. No. 68,532 filedNovember 10, 1960 which has now been abandoned.

The possibility of detecting the presence of a conveyance in a definedarea is evidenced by the various disclosures shown in the prior art.Most of such disclosures depend upon the presence of a conveyance in adetector loop associated with an oscillator for changing the frequencyof the oscillator to the extent where an accompanying circuit iscontrolled. More particularly, many such disclosures include anoscillator which is normally effective to hold a relay means in eitheran energized or deenergized condition, while the presence of aconveyance in the detector loop associated with the oscillator iseffective to cause the relay means to be operated to the oppositecondition where a suitable indication is given. The detection of aconveyance has been additionally attained by employing a plurality ofdetector loops which are normally electromagnetically coupled forcontrolling an associated indication means to one condition, while thepresence of a conveyance in any one of the detector loops is effectiveto decouple the plurality of loops in a manner to cause a diiferentindication to be rendered. In each instance, however, the'predominatingcharacteristic of holding the controlled circuit means in one conditionduring the absence of conveyances and causing the controlled circuitmeans to be operated to the opposite condition when a conveyance isdetected is present.

In the present invention, a presence detection system is proposed whichis effectively normally operated between two distinct conditions ofoperation at a predetermined rate with suitable indications beingprovided therefor. It is further proposed in this invention that thepresence of a conveyance in a defined area be efiective to cause one ofthe normal conditions of operation to be predominate for the intervalthat the conveyance is within the defined area. Moreover, the presentinvention proposes that a dstinctive indication be provided for each ofthe two distinct conditions under normal operation with a distinctiveindication other than the two distinct indications mentioned above beingprovided for indicating the presence of a conveyance within the definedarea.

In the present invention, several advantages are attained which areinherent with the character thereof as contrasted to that disclosed bythe prior art. In the operation of the system organization under normalconditions, i.e., when no conveyance is detected as being within thedefined area, appropriate indications are provided which are indicativeof this normal condition. When a conveyance is detected in the definedarea, however, the system organization is also operative to indicatethis condition. For other conditions of operation, i.e., where somecomponent of the system has failed to operate properly, the circuitorganization is adapted to provide an indication which is indicative ofthe presence of a conveyance within the defined area. Thus, failure ofthe circuit organization to operate properly provides an indication ofdetection even though a conveyance is not actually within the definedarea. Moreover, operation of the system orice ganization in this mannerprovides a means of checking that the system is operating and will beeffective to detect a conveyance which comes within the defined area.

Thus, one object of this invention is to provide a system organizationadapted to normally operate between two distinct conditions of operationat a predetermined rate with characteristic indications being providedfor each distinct condition of operation.

Another object of this invention is to provide a system organizationadapted to be interrupted from its normal operation according to thepresence of a conveyance in a defined area and for the interval therein.

Another object of this invention is to provide a system organizationwhich is normally operative between two distinct conditions of operationand which is adapted to operate to that condition of operation accordingto component failure wherein an indication of detection is provided.

Another object of this invention is to provide a system organizationwhich is capable of being controlled between its two distinct conditionsof normal operation by either an internal or external means.

Other objects, purposes and characteristic features of the presentinvention will be in part obvious from the accompanying drawings, and inpart pointed out as the description of the invention progresses.

In describing the invention in detail, reference will be made to theaccompanying drawings, in which like reference characters designatecorresponding parts through the several views, and in which:

FIG. 1 illustrates substantially by block diagram the present invention;

FIG. 2 illustrates by detailed circuitry the embodiment of the presentinvention;

FIG. 3 illustrates an alternate circuit for controlling conditionindicating means and means of internally controlling the normaloperation of this invention; and

FIG. 4 illustrates another manner of controlling the conditionindicating means and the means of internally controlling the normaloperation of this invention.

For the purpose of simplifying the illustrations and facilitating in theexplanation, the various parts and circuits constituting the embodimentof the invention have been shown diagrammatically and certainconventional illustrations have been employed, the drawings having beenmade more with the purpose of making it easy to understand theprinciples and mode of operation than with the idea of illustrating thespecific construction and arrangement of parts that would be employed inpractice. Thus, the various relays and their contacts are illustrated ina conventional manner, and the symbols and in addition to the symbols(3+) and (B) are used to indicate connection to the opposite terminalsof suitable batteries, or other sources of electric current, instead ofshowing all of the wiring connections to these terminals.

System apparatus With reference to FIGS. l-4, the apparatus employed inthe present invention may be generally described. A portion of a routelti'over which conveyances of different types may suitably travel andinto which a coil loop 11 is suitably embedded so as to be in the pathof conveyances travelling over route 10 is shown. The types ofconveyances adapted to travel over route 10 may include airplanes,vehicles including both cars and trucks, and other movable conveyanceseach of which is at least partially constructed of a material such asmetal for influencing the inductance of coil loop 11.

The coil loop 11 is connected to an oscillator 14 which is normallyoscillating at a frequency F1 and which may be changed to a frequency F2according to the presence of a conveyance with the confines of coil loop11. Another oscillator 15 is provided which also has the oscillatingfrequency F1 and which additionally is controlled to oscillate at afrequency F3 according to the control thereof by a frequency controlcircuit 17 or, alternately, the control thereof by a control relaycircuit 19. Respective outputs from oscillators 14 and 15 are coupledthrough capacitors 21 and 22 to a combination of circuit means providedfor the purposes of detecting any difference frequency existing betweenthe current oscillating frequencies of oscillators 14 and 15, amplifyingsuch detected difference frequency, rectifying the amplified differencefrequency signal, and employing such rectified direct current signals tocause corresponding direct current signals to be produced at the output.The direct current outputs are supplied to a control relay circuit 19provided for the purpose of responding to the conditions established bythe combination of circuit means. An indication circuit means whichincludes a relay 25 and indicating lamps GL, YL, and RL is controlledaccording to the responsive or nonresponsive conditions of control relaycircuit 19 and is provided for he purpose of indicating the normaloperation of the system organization in addition to providing anindication according to the presence of a conveyance Within the coilloop 11.

Each of the oscillators 14 and 15 is commonly referred to as aseries-feed Hartley oscillator. In each instance, a portion of the tankcircuit, the triode type tube and the power supply form a seriescircuit. For oscillator 14, this series circuit extends from (B+),through a portion of the coil loop 11, through the triode type tube T1,to (B), while for oscillator 15, the series circuit extends from (B+),through a portion of the tank coil 30, through the triode type tube T2,to (B). Under normal conditions, i.e., when no conveyance is Within thearea defined by coil loop 11, the frequency of oscillation 14 isdetermined by the values of coil loop 11 and at capacitor 23 whichcomprise the tank circuit for oscillator 14. The frequency ofoscillation for oscillator 15 is determined by the values of coil 3t)and a capacitor 31 which form the tank circuit for oscillator 15. Attimes, during normal operating conditions, the capacitor 28 iseffectively connected across a portion of the tank coil 30 to change thefrequency of oscillation for oscillator 15.

The capacitors Z1 and 22 are additionally employed to providesynchronism between oscillators 14 and 15. That is, each of thecapacitors 21 and 22 has a value such that the combination thereof iseffective to overcome electrical disturbances resulting from changingWeather conditions and/ or slight changes in component values to theextent that no noticeable difference frequency can exist betweenoscillators 14 and 15 for the oscillating frequency F1.

A frequency control circuit 17 is provided for the purpose of biasing adiode 27 which permits the capacitor 28 to be connected into and out ofcircuit relationship with tank coil 3% at a predetermined rate. Thefrequency control circuit 17 includes a free-running multivibrator ofespecial design which is adapted to at all times initiate operation withthe application of supply voltage. This insures that the capacitor 23 iseffective from the inception of circuit operation which will bediscussed in more detail hereinafter. In place of frequency controlcircuit 17 which may be considered an external control, internal controlmay be provided by control relay circuit 1& for connecting anddisconnecting a capacitor 32 into the tank circuit including tank coil31 and capacitor 31 for effecting a change of the frequency ofoscillation for oscillator 15.

Control relay circuit 19, normally emp oyed to be responsive to outputsprovided by the combination of circuit means discussed above, may alsobe employed to control the frequency of oscillation for oscillator 15during normal operating conditions. in this respect, a relay of theneutral type included with the control relay circuit 19 responsive tothe outputs provided by the circuit means is at times adapted in onecondition thereof to control the frequency of oscillation for oscillator15. A timing circuit is empolyed with this relay for establishing therate at which a controlled circuit is adapted to connect and disconnectthe capacitor 32 with tank coil 30. In another instance, two neutraltype relays are provided where one relay is controlled by the outputsfrom the circuit means and the other relay is responsive to theconditions of the one relay and additionally to a timing circuit forcontrolling the frequency of oscillation for oscillator 15.

The combination of circuit means provided for detecting any differencefrequency existing between the current oscillating frequencies ofoscillators 14 and 15, amplifying this difference frequency, rectifyingthe amplified difference frequency signal, and further employing theserectified direct current signals to provide outputs to the control relaycircuit 19 includes circuits for both effecting the function commonlyreferred to as heterodyning and for converting the amplified heterodynesignal to an amplified pulse output. The frequencies coupled fromoscillators 14 and 15 through capacitors 21 and 22 are supplied to adetector 35 where they are combined and where the difference frequency,if any, is detected. The difference frequency is then supplied to analternating current amplifier 36 where it is amplified with theresulting amplified signal then being supplied to a detector 37 which isprovided for the purpose of converting the alternating current signalcharacteristic of the difference frequency to a direct current signal.The direct current signal is then supplied to a direct current amplifier39 which is provided for the purpose of amplifying the direct currentsignals and further controlling the output supplied to control relaycircuit 19.

The indication circuit means which includes relay Z5 and indicatinglamps GL, YL and RL is provided for the purpose of indicating the normaloperation of the system organization and providing an indicationaccording to the presence of a conveyance within the area defined bycoil loop 11. Relay 25 is a neutral type relay and is adapted to remainin its energized condition under normal operating conditions of thesystem organization. Indication lamps GL and YL are adapted to beilluminated alternately according to the energized condition of relay 25with the alternate illumination thereof being indicative of propersystem operation. Indication lamp RL is adapted to be illuminatedaccording to the deenergized condition of relay 25 for being indicativeof the presence of a conveyance within the area defined by coil loop 11or that the system organization is malfunctioning.

It is believed that the nature of the invention, its advantages andcharacteristic features can be best understood with further descriptionbeing set forth from the standpoint of operation.

Operation Under normal conditions, i.e., when the area defined by coilloop 11 is devoid of conveyances, two conditions of operation normallyobtain. In one of these conditions of operation, both of the oscillators14 and 15 are adapted to oscillate at the frequency F1. During thesecond condition of operation, the frequency of oscillation foroscillator 15 is changed to frequency F3 for an interval as determinedby frequency control circuit 17 or, alternately, control relay circuit19. During these tWo conditions of operation, the indication lamps GLand YL are alternately illuminated and deenergized at a predeterminedrate which is indicative of normal system operation.

If it is assumed that both oscillators 14 and 15 have similarfrequencies of oscillation, certain circuit conditions prevail. Thediode 27 is effectively biased by frequency control circuit 17 so as toprovide that capacitor 28 is disconnected from the upper portion of tankcoil 30.

This biasedcondition of diode 27 'is effected according to the directcurrent output provided by frequency control circuit 17.

Referring to FIGS. land 2, it is seen that the frequency control circuit17 includes two PNP type transistors Q1 and Q2 with associated resistorsand capacitors which are adapted to comprise a free-runningrnultivibrator. The arrangement of the elements for circuit 17 is suchthat it will always assume one condition of operation with theapplication of supply voltage. For the present condition, however, itis'assurned that transistor Q1 is conducting and transistor Q2 is in anonconducting condition. Under these conidtions, a negative potential issupplied from through resistor 41, through resistor 42, through a radiofrequencychol e44 to the positive side of diode 27. Capacitor 28 is thusefiectively disconnectedfrom the upper portion of tank coil 30.

Each of the oscillators 14 and at this time are effectively oscillatingat the frequency F1. The outputs taken from the plate circuits of tubesT1 and T2 included with oscillators 14 and 15 respectively are suppliedto a first detector through a coupling capacitor 46. The action of thedetector which includes diodes 49 and 5t), capacitor 51 and a resistor53 provided 'for bleeding the charge from capacitor 51 is in'eifectiveto provide an output since the similar frequencies when combined cancel.The AC. amplifier 36 which comprises NPN type transistor Q3 and PNP typetransistor Q4 with associated resistors remain in the normallysemiconductive conditions as provided by the biasing conditions of theassociated resistors. Under these conditions of operation, noalternating current signal is provided at the output of amplifier 36which is normally taken from the positive side of resistor 57 andsupplied to detector 37 through a coupling capacitor 58. Detector 37which includes diodes 60 and 61, capacitor 63 and a resistor 64 providedfor the purpose of bleeding the charge from capacitor 63 is ineffectiveto provide an output to the DC. amplifier 39. Amplifier 39 whichincludes PNP type transistors Q5 and Q6 remains in its normally biasedcondition where transistor Q5 is in a nonconducting state according to apositive (1-) potential being supplied through resistor 64 to the basethereof and transistor Q6 is in a conducting state according to anegative potential being supplied through resistor M4 to the basethereof.

Indication lamp GL is caused to be illuminated during the normalconducting condition of transistor Q6 which provides that a relay 68included in circuit 19 and relay be energized. The circuit fortransistor Q6 during its conducting condition extends from through diode79 provided for the purpose of biasing transistor Q6, through transistorQ6 from the emitter to collector through the base thereof, through thewinding of relay 68, to In this conduction circuit, relay 63 isenergized. In the now energized condition of relay 68, relay 25 isenergized by a circuit extending from the positive side of capacitor 74,through front contact 76 of relay 68, through resistor 77, to thewinding of relay 25, to ground. Also, a capacitor 79 connected in shuntwith relay 25 is also charged at this time and is provided forcontrolling the release of relay 25 when relay 68 is deenergized underoccupancy detection circumstances. With relays 25 and 68 respectivelyenergized, lamp GL is illuminated by a circuit extending from throughfront contact 81 of relay 25, through front contact 86 of relay 68,through the filament of lamp GL, to

If it is assumed that the opposite normal condition prevails, i.e., thefrequency of oscillation for oscillator with circuit 17 must be in aconductive state, while transistor Q1 also included therewith must be ina nonconductive state.

During the conducting condition of tranisstor Q1 assurned above, theconducting circuit therefor extends from through resistor 83, throughtransistor Q1 from emitter to collector through the base thereof,through a resistor 84, to Capacitor connected in shunt with resistor 83is also charged during this circuit operation. The base circuit fortransistor Q1 is effectively biased by a circuit extending from throughresistor 87, through resistor 88, through resistor 41, to When capacitor85 becomesfully charged and subsequently initiates a discharge throughresistor 83, the current flow through transistor Q1 is'elfectiv'elydecreased to the extent that the bias provided for the base circuit oftransistor Q2 by a circuit extending from through resistor 90, throughresistor 91, through resistor 84, to is changed to a more negative-goingsignal according to the decreased voltage drop across resistor 84.Transistor Q2 is placed in a conducting condition and conducts throughthe circuit extending from through resistor 93 and capacitor 94connected in shunt, through transistor Q2 from emitter to collectorthrough the base thereof, through resistor 41, to A positive-goingsignal taken from the positive side of resistor 41 is supplied to thebase of transistor Q1 for causing the nonconduction thereof and issupplied to diode 27 through resistor 42 and radio frequency choke 44which causes diode 27 to be forward biased according to current flowingin coil 30.

The inclusion of capacitor 28 across the upper portion of tank coil 30is effective to change the frequency of oscillation for oscillator 15 tofrequency F3 and this frequency F3 Will be lower than frequency F1 asdetermined by the value of capacitor 28. The outputs coupled fromoscillators 14 and 15 through capacitors 21 and 22 detector 35 throughcapacitor 46 thus have different frequencies. Detector 35 is theneffective to combine the alternating current voltages of differentfrequencies in the usual manner so asto produce at its output adifference frequency signal including a direct current signal. Thedirect current signal is decoupled from the difference frequency signalby capacitor 95 and the diiference freqeuncy signal is then supplied tothe base of transistor Q3. This signal, being of an alternating currentnature and commonly referred to as a heterodyne signal, causestransistor Q3 to be operated between its conductive state andnonconductive state according to the values of the signal with respectto a base biasing circuit. Transistor Q4, being directly responsive tothe operation of transistor Q3, is operated between its conductive stateand nonconductive state in a manner to cause an amplified heterodynesignal to be supplied to detector 37. The amplified heter odyne signalis rectified by detector 37 and is adapted to provide a negative-goingoutput signal which is supplied to amplifier 39 for causing a reversalof operation for transistors Q5 and Q6.

During normal conditions, a biasing circuit for transistor Q3 extendingfrom through resistor 98, through resistor 99, to normally places thebase of transistor Q3 at a positive potential. Transistor Q3 is thenbiased into conduction with current flowing from through resistor 192,through transistor Q3 from its collector to emitter through the basecircuit thereof, through resistor 55, to A voltage drop across resistor102 places the base of transistor Q4 at a negative poter tial whichcauses the conduction thereof through a circuit extending from throughtransistor Q4 from its emitter to collector through its base circuitthereof, through resistor'57, through resistor 55, to With theheterodyne signal being supplied to the base circuit of transistor Q3for the interval that oscillator 15 is oscillating at frequency F3, thebiasing conditions thereof are changed to the extent that transistor Q3conducts and nonconducts according to the signal. Similarly, transistorQ4 is made to follow the operation of transistor Q3 in- 7 asmuch as itsbiasing circuit is included in the conducting circuit of transistor Q3.An amplified version of the heterodyne signal is then produced at thepositive side' of the resistor 57 which is coupled into detector 37having diodes 60 and 61 and capacitor 63 and which is effective to actupon the amplified heterodyne signal to the extent that capacitor 63 ischarged to twice the voltage level of such amplified heterodyne signal.The negative side of capacitor 63 being connected to the base oftransistor Q causes this transistor to become conductive through acircuit which extends from through transistor Q5 from its emitter tocollector through its base thereof, through resistor MP4, to TransistorQ6 having its base connected to the positive side of resistor 194 isbiased to a nonconductive condition as transistor Q5 conducts and forthe interval thereof. As mentioned above, the energizing circuit forrelay 6% is dependent upon the conducting condition of transistor Q6. Inthe nonconductive condition of transistor Q6, the collector circuitthereof is placed at so as to cause relay 63 to be deenergized.

In the deenergized condition of relay 68, the energizing circuit forrelay 25 described above as including front contact 76 of relay 68 isdisconnected. Relay 25 will remain energized, however, as capacitor 79is discharged therethrough until relay 68 is once again energized forreconnecting the circuits described above. In the deenergized conditionof relay 68, lamp YL is illuminated by a circuit extending from throughfront contact 81 of relay 25, through back contact 80 of relay 68,through the filament of lamp RL, to The illumination of lamp YL is thenindicative of the condition where oscillator is oscillating at thefrequency P3.

In lieu of the external control provided by frequency control circuit17, two additional means of providing internal control of oscillator 15are illustrated in FIGS. 3 and 4.

Referring to FIG. 3, the relay 68 is shown to have a capacitor 106connected in shunt therewith as well as a resistor 197 connected inseries relationship. This resistor-capacitor combination is employed toprovide the timing control of relay 68 to its deenergized conditionafter transistor Q6 is placed in a nonconductive condition. During theenergized condition of relay 68, however, capacitor 32 is effectivelyconnected to point P as shown in FIG. 2 with the tank circuit ofoscillator 15 which includes tank coil 31 and capacitor 31 through acircuit extending from through front contact 198, through capacitor 32,to point P. In this connection, the frequency of oscillation asdetermined by capacitors 31 and 32 and tank coil 30 is effectivelychanged and this frequency is lower than frequency Fll. When relay 68 isdeenergized, this circuit is disconnected which causes the frequency ofoscillation for oscillator 15 to be at frequency F1.

Referring to FIG. 4, another relay 110 is employed to be responsive tothe conditions of relay 63 and is effective to control the operation ofrelay 25 as well as the connection of capacitor 32 to point P. In thisillustration the action of the circuit organization is SOI118\ hatfaster than that shown in FIG. 3 in that the action of relay 6% is notretarded in the manner shown in FIG. 3 with the inclusion of capacitor196. During the energized condition of relay 68, relay llltl isenergized by a circuit extending from through resistor 112, throughfront contact 113 of relay 68, through the winding of relay 116) toground. A capacitor 114 connected in shunt with relay Ht) is alsocharged during this operation. In the deenergized condition of relay 63,capacitor 114 is dis charged through back contact 113 of relay 6% andresistor 115 with the relay 11% being deenergized when insufficientcurrent flows through the winding thereof according to the discharge ofcapacitor 114. Relay 25 is controlled by the charge on capacitor 74 aswell as the charge on a capacitor 117 which is provided by a circuitextending M from through back contact 119 of relay 116, through aresistor 12%), through capacitor 117, to ground. The sustainedenergization of relay 25 under normal system operation causes the lampsGL and YL to be alternately illuminated according to the energizingcircuits therefor being completed through front contact 81 of relay 25and the front and back contact 82 of relay 110.

If it is assumed that at least one conveyance comes Within the areadefined by coil loop 11, the inductance thereof is influenced to theextent that the frequency of oscillation for oscillator 14 is changed tofrequency F2. During the interval that such conveyance is within theconfines of coil loop 11, this frequency of oscillation F2 prevails. Thefrequency of oscillation for oscillator 15 may be at frequency F1 orfrequency F3 according to the type of control employed for oscillator15, i.e., either frequency control circuit 17 or the control relaycircuits 19 as shown in FIGS. 3 and 4.

Irrespective of the control employed to change the frequency ofoscillation for oscillator 15, a difference frequency is detected bydetector 35, amplified by amplifier 36, rectified by detector 37 andemployed by amplifier 39 to control relay 68 to a deenergized conditionfor the interval that a conveyance affects the frequency of oscillationfor oscillator 14. The circuits 35, 36, 37 and 39 are responsive similarto the manner described above even though the difference frequencyresulting from the frequencies being combined may be somewhat differentthan occurs under normal conditions. As long as this differencefrequency prevails and relay 25 is effectively deenergized, the lamp RLis adapted to be illuminated through a circuit including back contact 81of relay 25 with the illumination thereof being indicative of aconveyance being detected within the coil loop 11.

To insure that each of the oscillators 14 and 15 are initiated intooperation with the supply voltage provided thereto, grid-leak biascircuits are employed. For oscillator 14, a grid resistor 124 isemployed in shunt with a grid capacitor 125 in order to provide that thecoil loop 11 is somewhat insensitive to extraneous objects. That is,with this grid-leak bias circuit arrangement, more grid drive isrequired in order to cause tube T1 to become conductive. On the otherhand, grid resistor 127 is connected from the grid to the cathode oftube T2 while capacitor 128 is connected in the grid circuit of tube T2in the usual manner. In the operation of tube T2, capacitor 130 isprovided for the purpose of filtering out radio frequency currents.

Having described a presence detection system and alternate formsthereof, as specific embodiments of the present invention, it is desiredto be understood that these forms are selected to facilitate in thedisclosure of the invention rather than to limit the number of formswhich it may assume; and, it is to be further understood that variousother modifications, adaptations and alterations may be applied to thespecific forms shown to meet the requirements of practice without in anymanner departing from the spirit or scope of the present invention.

What I claim is:

1. Apparatus for detecting the presence of motor vehicles, metallicobjects and the like, as they pass through a detection zone comprising:

(a) a first oscillator having a coil so positioned that each saidvehicle is in an inductive coupling relationship with said coil while insaid zone,

(b) means controlling said oscillator normally to oscillate and energizesaid coil with electrical energy of a first frequency Fl when no vehicleis in said detection zone and alternatively to oscillate and energizesaid coil with electrical energy at a frequency F2 when a vehicle is insaid detection zone,

(0) a second oscillator controlled at times to oscillate at said samefrequency F1 and at other times to 9 oscillate at a frequency F3different from both F1 and F2,

(d) oscillator control means connected to said second oscillator forrepetitively changing its operating frequency between said frequenciesF1 and F3,

(e) output means connected to both said oscillators and responsive tothe difference in frequency of the signals generated respectivelythereby for indicating the presence of a vehicle in said detection zonewhen continuously throughout at least a predetermined interval adifference in frequency exists between the outputs of said first andsecond oscillators.

2. The apparatus of claim 1 which further includes indication meansconnected to said output means and providing distinctive indication onlywhen intermittently and successively said oscillators operate at thesame and different frequencies.

3. The apparatus of claim 1 wherein said output means controls saidsecond oscillator to operate at frequency F3 when said first and secondoscillators are detected as operating on the same frequency F1 andcontrols said second oscillator to operate at frequency F1 when saidfirst and second oscillators are detected as operating on differentfrequencies.

4. The apparatus of claim 1 which further includes means coupling saidfirst oscillator to said second oscillator to thereby cause said firstoscillator to operate at exactly the frequency F1 of said secondoscillator when no vehicle is within said detection zone.

5. The apparatus of claim 1 in which said predetermined interval exceedsthe maximum interval that said second oscillator operates at itsfrequency F3.

6. The apparatus of claim 1 wherein said oscillator control meansincludes multivibrator means for controlling said second oscillator toalternately operate at its two respective frequencies at a ratedetermined by the frequency of said multivibrator.

7. The apparatus of claim 6 in which said second oscillator includes aresonant tank circuit, a reactance element adapted to be connected attimes in circuit with said tank circuit to change its frequency ofresonance, a diode connected in series with said reactance element,means for normally back-biasing said diode, said multivibrator meanswhen in one of its two conditions overcoming said back-bias on saiddiode.

References Cited by the Examiner UNITED STATES PATENTS 1,717,630 6/29Schaifer 340-258 2,390,221 12/45 Lindsay 340-258 2,421,771 6/47 Browning340-258 2,428,290 9/47 Peck 340-258 2,454,687 11/48 Baughman 340-2582,499,177 2/50 Baughman 340-258 2,537,298 1/51 Baughman 340-2582,558,445 6/51 Laurenson 340-258 2,646,559 7/53 Nutzler 340-2582,652,551 9/53 Gumpertz 340-258 2,721,994 10/55 Brown 340-258 2,917,73212/59 Chase et a1. 340-258 FOREIGN PATENTS 719,179 11/54 Great Britain.

NEIL C. READ, Primary Examiner.

1. APPARATUS FOR DETECTING THE PRESENCE OF MOTOR VEHICLES, METALLICOBJECTS AND THE LIKE, AS THEY PASS THROUGH A DETECTION ZONE COMPRISING:(A) A FIRST OSCILLATOR HAVING A COIL SO POSITIONED THAT EACH SAIDVEHICLE IS IN AN INDUCTIVE COUPLING RELATIONSHIP WITH SAID COIL WHILE INSAID ZONE, (B) MEANS CONTROLLING SAID OSCILLATOR NORMALLY TO OSCILLATEAND ENERGIZE SAID COIL WITH ELECTRICAL ENERGY OF A FIRST FREQUENCY F1WHEN NO VEHICLE IS IN SAID DETECTION ZONE AND ALTERNATIVELY TO OSCILLATEAND ENERGIZE SAID COIL WITH ELECTRICAL ENERGY AT A FREQUENCY F2 WHEN AVEHICLE IS IN SAID DETECTION ZONE, (C) A SECOND OSCILLATOR CONTROLLED ATTIMES TO OSCILLATE AT THE SAME FREQUENCY F1 AND AT OTHER TIMES TOOSCILLATE AT A FREQUENCY F3 DIFFERENT FROM BOTH F1 AND F2, (D)OSCILLATOR CONTROL MEANS CONNECTED TO SAID SECOND OSCILLATOR FORREPETITIVELY CHANGING ITS OPERATING FREQUENCY BETWEEN SAID FREQUENCIESF1 AND F3, (E) OUTPUT MEANS CONNECTED TO BOTH SAID OSCILLATORS ANDRESPONSIVE TO THE DIFFERENCE IN FREQUENCY OF THE SIGNALS GENERATEDRESPECTIVELY THEREBY FOR INDICATING THE PRESENCE OF A VEHICLE IN SAIDDETECTION ZONE WHEN CONTINUOUSLY THROUGHOUT AT LEAST A PREDETERMINEDINTERVAL A DIFFERENCE IN FREQUENCY EXISTS BETWEEN THE OUTPUTS OF SAIDFIRST AND SECOND OSCILLATORS.